In the prior art of molecular beam epitaxy (hereafter abbreviated as MBE), binary and ternary compounds have been grown. Two prior art U.S. Pat. No. 3,615,931, issued to John R. Arthur, Jr. on Oct. 26, 1971 and assigned to the assignee of this application and U.S. Pat. No. 3,751,310, issued to Alfred Yi Cho on Aug. 7, 1973 and assigned to the assignee of this application, are incorporated in this application by reference.
A binary compound, GaAs, has been formed in as apparatus that used GaAs feedstock in an oven. In that case, the ratio of Ga to As in the atomic beams was not critical because the formation of GaAs is self-regulating; the only requirement is that there be an excess of As. This property of self-regulation does not apply to many compounds, however, and in particular does not apply to pairs of elements from the same column of the Periodic Table.
When ternary compounds, such as Al.sub.x Ga.sub.1-x As or Ga.sub.x As.sub.1-x P, were formed, more than one oven was used and the value of x was determined by the relative temperatures of the ovens.
The prior art has used dopants, such as Ge in molecular beam epitaxy. A dopant is distinguished from an alloying element in that it is used to make a semiconductor compound n or p-type, is present in concentrations typically less than 10.sup.-4, and may vary in concentration by four orders of magnitude. In contrast, alloying elements are present in comparable amounts and must be controlled in concentration ratio to less than 1 percent in order to form a lattice-matched epitaxial layer onto a suitable substrate. The use of separate ovens for two or more elements that were required to be in a certain concentration ratio resulted in considerable variation of their ratio over the surface and throughout the thickness of the layer being formed, as a result of the different spatial distribution of the beams from the ovens and of fluctuations in the desired temperatures of the ovens.